X gate

The X gate, the quantum NOT, flips a qubit: $\vert 0 \rangle$ becomes $\vert 1 \rangle$ and $\vert 1 \rangle$ becomes $\vert 0 \rangle$. It acts on one qubit and takes no parameters. The Circuit panel draws it as X. Measured straight after it, a qubit that started as 0 reads 1.

Kets such as $\vert 0 \rangle$ name the basis states and rotations are described on the Bloch sphere; the qubits lesson introduces both.

Call

Matrix

On the Bloch sphere this is a half-turn about the X axis. It is the same operation as rx(π) up to a global phase.

Effect on basis states

On a superposition it swaps the two amplitudes, so $\vert + \rangle$ is unchanged and $\vert - \rangle$ only picks up a sign.

Inverse

X is its own inverse. Two X gates in a row on the same qubit cancel.

Usage

from qiskit import QuantumCircuit

qc = QuantumCircuit(1, 1)
qc.x(0)
qc.measure(0, 0)

import { QuantumCircuit } from 'qiskit';

const circuit = QuantumCircuit(1, 1);
circuit.x(0);
circuit.measure(0, 0);

Every shot reads 1.

On IonQ hardware

X is exactly GPi(0), a native gate, so it costs one physical pulse. See How your circuit is compiled.

Related

• CX gate
• √X gate
• RX gate
• Gate reference